FIG. 1 illustrates an exemplary fuel cell. This fuel cell 101 is a flat plate solid oxide fuel cell, and generates electric power by using a fuel gas such as a town gas, water, and air as an oxidizing agent. The fuel cell 101 includes an evaporator 102, an air preheater 103, a reformer 104, a burner 105, a cell stack 106, and other elements.
The evaporator 102 heats water to generate a water vapor. The generated water vapor is mixed with a fuel gas that is, e.g., desulfurized, and is subsequently sent to the reformer 104. In the reformer 104, the mixed gas of the fuel gas and the water vapor is reformed into a high temperature reformed gas primarily composed of hydrogen, and is subsequently supplied to the cell stack 106.
After being heated in the air preheater 103, the air is supplied to the cell stack 106. During a stationary operation, the supply of a high temperature reformed gas and air allows the cell stack 106 to be held in a predetermined operating temperature range between 700° C.-900° C.
The cell stack 106 includes a cell stack unit made by stacking a plurality of single cells formed in thin plate shapes. In the cell stack 106, an electromotive force is generated through a chemical reaction in each single cell while, at a high operating temperature, a reformed gas passes through an anode of the single cell and air passes through a cathode of the single cell. The electromotive force is extracted from each single cell to generate electric power.
Such a cell stack unit is disclosed in, e.g., Patent Document 1.
The altered, high temperature reformed gas and air exhausted from the cell stack 106 are sent to the burner 105. In the burner 105, the reformed gas and the air are mixed to cause combustion, which heats the reformer 104. An exhaust gas generated in the burner 105 is sent to the air preheater 103 to heat air by a heat exchange, and is subsequently exhausted.
Because the chemical reaction that allows for generating an electromotive force is an exothermic reaction, each single cell generates heat once the generation of electric power has been started. Thus, during a stationary operation, a cooling control that allows for a reduction in a temperature of air introduced to the cell stack 106 is implemented to hold the cell stack 106 in an operating temperature range.
Specifically, a bypass path 108 that bypasses the air preheater 103 is provided on an oxidizing agent gas introduction path 107 that introduces high temperature air into the cell stack 106 through the air preheater 103. Then, the air heated by the air preheater 103 is mixed with cool air, through the bypass path 108, with the flow rate thereof controlled by a flow rate control valve 109, in order to adjust a temperature of the air introduced to the cell stack 106.
An electric power generating system that implements a control similar to the above is disclosed in, e.g., Patent Document 2.